1. Field of the Invention
The present invention relates generally to object imaging methods, and in particular to an improved method for pictorially displaying output information generated in the form of an electrical signal by an object imaging apparatus.
2. Description of the Prior Art
Methods for pictorially displaying the output information of an object imaging apparatus are known in the art for use in conjunction with a scanning electron microscope. See the Journal of Physics E4, No. 11, pp. 837-842 (1971). In such methods, a working image of an object is generated on a first television monitor and an orientation image is generated on a second television monitor. The orientation image is generated by changing the operating parameters of the object imaging apparatus and is displayed simultaneously with the working image.
An electron microscope basically has two modes of operation. In one of the modes, a direct image of a segment of a specimen is generated by utilizing the usual scanning techniques and is reproduced on a television monitor. In the other of the modes, a channelling diagram of one point of the specimen segment displayed on the television monitor is generated and then reproduced on the second television monitor. Between these two modes of operation, the operating parameters of the electron microscope are switched at the frame frequency of the television monitors (which, for example, in Europe, is 50 Hz). The working and orientation images are, thus, generated alternately on the two display monitors.
In order to display the working and orientation images on the two television monitors without flickering, the switching of the operating parameters of the microscope between the described modes of operation must be carried out as frequently as possible. However, because of equipment limitations, there are also limits as to how frequently the switching can be carried out. For example, in an apparatus which is described in the above-referenced Journal of Physics pages, switching of the operating parameters of the microscope is carried out by varying the excitation of the magnetic lens of the microscope which is disposed after the microscope deflection system along the beam path. Since such magnetic lenses are generally constructed in the form of iron coils, variation of the excitation of the lens is almost always accompanied by eddy current phenomena.
The switching between the operating modes and the microscope may also be carried out at the line (horizontal) frequency of the television monitors. See Proceedings of the Fifth Annual Scanning Electron Microscope Symposium, IIT Research Institute, Chicago, Ill., pp. 49-56 (April 1972). In commercially available television monitors with 625 lines, this line frequency would be 31,250 Hz. By varying the excitation of the deflection systems of the monitors, images of different magnification may be generated on the television monitors. The monitor with the greater magnification will then reproduce a segment of the image with the smaller magnification. This segment is identified by a marker window disposed within the image. However, because of the equipment limitations previously mentioned, only limited variation of the mode of operation of the microscope can be achieved with such a method. Thus, for example, all variations which require variation of the excitation of one or more magnetic lenses are eliminated. To implement such variations, known methods require considerable electronic circuitry to be used since the switching from one mode of microscope operation to the other must be performed at the frame or line frequency of the television monitors. Moreover, the switching must be performed at short time intervals in order to avoid flickering of the displayed working and orientation images. As a result, known methods for comparative observation of the working and orientation images can be used only if the changes in the operating parameters of the imaging apparatus can be carried out in an extremely short period of time. It is, accordingly, impossible, for example, to compare the images before and after inserting a diaphragm into the imaging apparatus.